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'[PIC]: Shared serial bus'
2003\01\19@153923 by Igor Pokorny

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Hi all,

I have plenty IR receiver/transmitter units connected to a serial bus. Units
can be initiate in any moment to send message to another (addressed) unit by
a serial bus. Does someone know how to arbitrate  collisions in a serial bus
easily? There isn't any master in a system. Every unit has a buffer to save
whole message. I tried to find any protocol but haven't found any.

Thanks

Igor

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2003\01\19@155809 by Wouter van Ooijen

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> Does someone know how to arbitrate  collisions
> in a serial bus easily?

I don't know the proper name, but the principle is that each node has a
number. During arbitration each station that wants to send puts his
number on the bus, but it looses when the (open-collector type) bus does
not show the bit it is putting on the bus. This ensures arbitration
between N stations in log(N) bits. Add rotation to achieve fairness,
when needed. I'm not sure whether you will call this simple....

Wouter van Ooijen

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2003\01\19@161844 by Herbert Graf

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> Does someone know how to arbitrate  collisions
> in a serial bus easily?

I don't know the proper name, but the principle is that each node has a
number. During arbitration each station that wants to send puts his
number on the bus, but it looses when the (open-collector type) bus does
not show the bit it is putting on the bus. This ensures arbitration
between N stations in log(N) bits. Add rotation to achieve fairness,
when needed. I'm not sure whether you will call this simple....

Wouter van Ooijen

       Perhaps overkill but perhaps one could consider a token ring system. It
isn't that hard to implement and almost guarantees no colisions. TTYL

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2003\01\19@175413 by Igor Pokorny

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We use method descripted by Wouter sharing 485 bus, but there is a master
that initiate the process.

Token ring sounds better, but there is a question of power consumpting.
PIC is awaken every 20 miliseconds for 200 uS to catch IR pulse or by
activity of a serial line. I forgot to notice, that only a few units
communicating with controlled equipment are addressed only. I would like to
keep the system without a need to configure it.

{Original Message removed}

2003\01\19@191109 by Olin Lathrop

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> I don't know the proper name, but the principle is that each node has a
> number. During arbitration each station that wants to send puts his
> number on the bus, but it looses when the (open-collector type) bus does
> not show the bit it is putting on the bus. This ensures arbitration
> between N stations in log(N) bits. Add rotation to achieve fairness,
> when needed. I'm not sure whether you will call this simple....
>
> Wouter van Ooijen

That is the way CAN arbitration works.  If one node is more a "controller"
and the other "slaves", then you can use master arbitration.  One scheme is
the master gives each slave a chance to communicate in round-robin fashion.
The slave either communicates or indicates it has nothing to say so that the
master can quickly address the next slave.  I used this scheme successfully
on an RS-485 network controlling a bunch of separate PICs in a flight
simulator system once.  CAN would be a better choice for this project today.


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